It is known that certain classes of semiconductor devices consume substantial amounts of power, which results in excess thermal energy that then must be transferred to the ambient environment. This waste heat is typically communicated through a variety of thermal interfaces, heat spreaders, and structural elements prior to being rejected into the ambient atmosphere by a heat sink. Since heat is often dissipated to room temperature air, and the silicon constructed semiconductor has a finite upper bound on its operating temperature, package-related thermal resistance is becoming a limiting factor in the ability to dissipate the waste heat.
The removal of package elements and interfaces will reduce package thermal resistance, and allow the semiconductor device to either run cooler or dissipate more power. However, many of these elements are required in order to provide a match between the relatively low coefficient of thermal expansion (CTE) of silicon and the relatively high CTE of the metal comprising the heat sink, rather than for best thermal performance. This match needs to be maintained in order to prevent build-up of stress, as well as subsequent damage due to failure of the relatively brittle silicon component. Thus, there are the competing structural requirements of providing a layer of material to provide a CTE match while at the same time needing to bring the heat transfer structure into intimate physical contact with the heat generating structure.
Matching may be achieved by at least two methods: the use of an alloy substrate such as copper/tungsten whose CTE matches or nearly matches that of the silicon, or through the use of a ductile braze alloy between the silicon and the remaining package elements. Either method prevents transmission of stresses due to mismatched CTE through the interface to the silicon device. Some disadvantages of the alloy substrate include expense, unfavorable machining and stamping characteristics, and a fairly low thermal conductivity. Some disadvantages of the ductile braze alloy include a limited fatigue life, which eventually results in failure due to delamination of the joint. This tendency is exacerbated by the service conditions of most high power devices. Such devices almost always operate under conditions of periodic fluctuating electrical load, which leads to periodic fluctuations in thermal load and mechanical stresses in the joint.
An alternative method involves the use of direct bond copper (DBC) aluminum nitride (AlN) in sheet form. This material is a “sandwich” comprised of a single layer of aluminum nitride and two outer layers of OFE copper foil. The copper layers are first oxidized, and then pressed against the AlN at high temperature in a neutral atmosphere. This process causes the oxide to diffuse into the AlN and bonds the copper sheets tightly to the AlN inner layer. Since the copper layers are relatively thin and are in an annealed state due to the high processing temperature, the CTE of the resulting assembly is largely governed by that the of the AlN.
None of the foregoing techniques have been found to be completely satisfactory or have been successfully applied to heat pipe cooling devices.